It's not "only three levels". All my irons have had "continuous" "level" knob, with three "presets", represented as dots. Along with other "presets" for different kinds of things to iron. But maybe that's Euro thing.
Funny thing, those two do not match. For example, on my current iron "Cotton" is marked at three-dots, whereas all my cotton clothes have labels that insist on ironing them on two-dots.
Just to highlight how non-universal these symbol are, I have never had an iron that has three levels of heat or that represents the levels with dots.